Field of the Invention and Related Art Statement
The present invention relates to an optical information recording and reproducing apparatus in which an actual positional error signal can be obtained by eliminating an offset component.
In optical information recording and reproducing apparatuses, a light beam is made incident upon an optical or opto-magnetic information record medium by means of an optical pick-up and an information is reproduced by detecting light reflected by the record medium. As the optical pick-up, there has been recently developed an optical pick-up of separation type in order to access a given information track in the record medium at a high speed.
The separation type optical pick-up comprises a stationary unit having a light source and several optical elements for detecting focusing error, tracking error and information signal, and a movable unit including reflecting member and objective lens for projecting a light beam emitted by the light source onto a desired position on the information record medium. In this manner, the optical system is divided into the stationary and movable units, so that the assembling and adjustment of the light source become easy and further the focusing control can be performed very precisely by adjusting the objective lens provided in the movable unit or the movable unit as a whole. It should be noted that the tracking control can be performed easily by moving the reflecting member or the objective lens or the whole movable unit. When the light spot is to be projected onto a given information track, the light spot is moved over a wide range on the record medium by crossing a plurality of information tracks. This mode of operation is usually called the seek. For performing the seek operation, the movable unit is moved in a direction perpendicular to the information track. That is to say, in case of using an optical information record disk, the movable unit is moved in the radial direction of the disk, i.e. the tangential direction of the circular information track.
The inventor of the instant application has confirmed that in the known optical information recording and reproducing apparatus comprising the separation type pick-up, the focusing control could not be performed accurately due to various factors. For instance, when an optical axis of a light beam emanating from the stationary unit is inclined with respect to an axis along which the movable unit is driven, when the position of the movable unit varies in the tangential direction of the optical disk in dependence on a position on the radial direction of the disk due to a mechanical error, when the optical axis is deviated due to the thermal property of various portions of the optical information recording and reproducing apparatus, when the optical record disk has an inclination in the tangential direction, when the inclination of a turntable rotated by a spindle motor is changed with respect to the optical system due to the secular variation, and when the disk groove crossing signal is leaked into the tangential direction, there is introduced an offset in the focus error signal, and thus the focusing error could not be detected accurately and thus the focusing control could not be performed precisely.
FIG. 1 is a schematic view for explaining the manner of generating the deviation in the tangential direction when the optical axes of the movable and stationary units are inclined with respect to the axis along which the movable unit is driven. When an optical system arranged in a movable unit 20 has an optical axis which is inclined with respect to an axis A along which the movable unit is moved, optical axes of light beams emanating from the movable unit are deviated in the tangential direction by an amount .delta. on the optical system of the stationary unit 21. That is to say, when the movable unit 20 is moved in the radial direction of the optical information record disk from the position shown by a solid line in FIG. 1 to a position shown by a chain line, a position of the stationary unit 21 at which the light beam emanating from the movable unit is received is shifted in the tangential direction. In FIG. 1, a reference numeral 22 denotes a position of the spindle motor. Due to this deviation in the tangential direction, the light amount distribution impinging upon a focus error detecting optical system is deviated in the tangential direction, so that the detected focus error signal includes the offset.
In Japanese Patent Application Laid-open Publication Kokai Sho 61-182640, there is described one solution for eliminating the offset due to the deviation of the optical axis of the pick-up of separation type. In this known solution, the deviation of an incident angle of a light beam impinging upon the reflecting member in the movable unit is detected and a mutual positional relationship between the stationary unit and the movable unit is adjusted in accordance with the detected deviation of the incident angle by means of, for instance, a yawing mechanism. To this end a photo-detector having two light receiving regions for detecting the deviation in the incident angle and a beam splitting means for introducing a light beam into the photodetector have to be arranged on the movable unit. Therefore, the weight of the movable unit becomes heavy and the seek property becomes worse. That is to say, the movable unit could not be moved in the radial direction of the optical information disk at a high speed for effecting the seek operation.
In Japanese Patent Application Laid-open Publication Kokai Sho 61-14863, there is disclosed another known solution for removing the offset in the focus error. In this known solution, the light beam is divided into two beams in the actuator, i.e. movable unit, and one of beams is used as a usual beam for effecting the recording, reproducing and erasing. The other beam is reflected within the actuator and the thus reflected light beam and the light beam reflected by the optical record disk are traveled along the same optical path toward the stationary unit. In the stationary unit, these beams are separately detected to derive a signal representing the offset of the focus error. In this known solution, beam splitter, a quarter wavelength plate and reflection mirror have to be provided in the movable unit, so that the movable unit becomes heavy and thus the seek property is deteriorated. Further the beam splitter for introducing the light beam into the photodetector for detecting the deviation of the optical axis is arranged in the stationary unit, a large amount of P-polarizing component returns to the light source.